A) Field of the Invention
The present invention relates to a semiconductor optical device and more particularly to a semiconductor optical device having quantum dots in an active layer.
B) Description of the Related Art
Semiconductor devices are known which utilize quantization. As the thickness of a semiconductor layer becomes thin, one-dimensional quantization is possible. Semiconductor lasers and the like utilize a multi-quantum well (MQW) structure made of an alternate lamination of one-dimensionaly quantized well layers and barrier layers.
Some logical devices and the like utilize quantum wires two-dimensionally quantized by restricting the thickness and width of a semiconductor layer. Some logical devices and light emission devices utilize quantum dots three-dimensionally quantized by restricting all the sizes of a semiconductor layer along three-dimensional directions.
As material not lattice-matching an underlying substrate, i.e., material which generates strain, is epitaxially grown, micro fine islands in the order of several nm to several tens nm are grown spontaneously in some stage of the initial growth period (S-K mode). Quantum dots have been formed conventionally by utilizing this phenomenon.
Quantum dots are formed by epitaxially growing on a GaAs or InP substrate InAs or InGaAs (for InP substrate, In-rich InGaAs) having a narrower band gap and a larger lattice constant than those of the substrate. In practice, semiconductor barrier layers lattice-matching a substrate and quantum dots having a larger lattice constant and a narrower band gap are alternately laminated. This forms the structure that quantum dots having a narrow band gap are buried in the barrier layers. Compressive stress is generated in quantum dots in such a device.
As strain is generated in (as stress is applied to) a semiconductor crystal, a hole level degenerated at the band edge is split into a heavy hole level and a light hole level. In a quantum dot with compressive strain, energy difference between electron and heavy hole is smaller than energy difference between electron and light hole. In this case, interaction with transverse electric (TE) mode light becomes dominant. An optical amplifier or a laser device formed by using quantum dots with compressive strain amplifies or generates TE mode light.
A semiconductor optical device is desired which generates or amplifies transverse magnetic (TM) mode light. Such a semiconductor optical device using quantum dots is not known as yet.
In a photonic network or the like utilizing optical fibers, the polarization direction of propagating light changes and is infinite. One of the requirements of an optical amplifier is to amplify both TE mode light and TM mode light. It is not sufficient if an optical amplifier amplifies only TE mode light.
In conventional semiconductor optical devices utilizing quantum dots, interaction with TE mode light has been dominant. Quantum dots of conventional semiconductor devices have been quantum dots having compressive strain.